There is a continuing demand for improved types of coating compositions, both solvent-based and water-reducible. More specifically, there is a continuing and growing demand for water-reducible, sprayable compositions suitable for application to metal surfaces intended to come in contact with beverages, and especially, for lining the interior of beverage cans.
Of the patent applications identified above, Ser. Nos. 685,246, 788,611 and 793,507 describe novel, sprayable, water-reducible coating compositions that are particularly useful for lining beverage cans. The compositions described have superior metal-coating characteristics derived from an epoxy resin component, and economy contributed by a vinyl resin component. These important characteristics are available, moreover, in compositions that when cured meet the many demanding tests with which any beverage can lining composition is confronted.
Nevertheless, the need for continuing improvement in functional characteristics, in economy, and in properties that facilitate compliance with environmental protection legislation, particularly with respect to the release of solvents to the atmosphere, indicate the need for still further technological change and progress.
The first-filed application above, Ser. No. 685,246, in one preferred embodiment, discloses a process for preparing a curable resinous composition having an Acid Number of at least 30, by reacting together at 90.degree. C. to 130.degree. C. an aromatio 1,2-epoxy diepoxide resin having a molecular weight above 350 and addition polymerizable monomer of which from 10% to 80% by weight is an acrylic acid, the diepoxide resin being present in sufficient quantity to provide from 10% to 90% by weight of the initial reaction mixture, in the presence of a free radical initiator of the benzoyl peroxide type. During the reaction there is simultaneous addition polymerization of the monomer through its ethylenic unsaturation and grafting of addition polymer to the epoxy resin. The graft polymer is characterized by the substantial absence of hydrolyzable functional groups. The ionizability of the reaction mixture, by reason of its acid functionality, is sufficiently high to effect stable dispersion of the product in an aqueous ionizing medium.
In a preferred embodiment, an aromatic diepoxide such as a polyglycidyl ether of bisphenol A is reacted with a mixture of addition polymerizable monomers containing a major amount of methacrylic acid. The epoxy resin has a molecular weight of at least 1,000, and provides from 30% to 90% of the initial reaction mixture. The reaction takes place in the presence of benzoyl peroxide at a temperature of 90.degree. C. to 130.degree. C., to effect addition polymerization of the monomer and to produce a graft polymer of addition polymer to the diepoxide having an Acid Number of 30 to 150 or more, preferably 70-90. The reaction product may be dispersed in a basic aqueous medium, to form a water-reducible coating composition. Generally a cross-linker is added, such as an aminoplast, and curing is effected in an oven. Most preferably the epoxy resin has a molecular weight in the range 4,000 to 10,000 and provides 50% to 90% of the initial reaction mixture.
As is more particularly pointed out in patent application Ser. No. 788,611, the resinous reaction product produced contains three polymeric components, namely, the graft polymer, ungrafted epoxy resin, and ungrafted addition polymer.
As is pointed out in Ser. No. 793,507, the initial epoxy resin, that is employed in the graft polymer production process, may be terminated to eliminate part or all of the terminal epoxy groups, to eliminate ester grafting at the terminal epoxy groups. Elimination of the terminal epoxy groups also permits the efficient use of a wider variety of peroxide-type free radical initiators, over a broader reaction temperature range, than would otherwise be the case.
As is disclosed in these prior applications, in order to make acceptable water-reducible coating compositions, the addition polymerizable monomer comprises a major proportion of an unsaturated carboxylic acid, preferably either acrylic or methacrylic acid. Sufficient acid is employed so that the Acid Number (NV, i.e., based on non-volatiles) of the reaction product is from about 30 to 200. The ionizability of compositions prepared in accordance with these several patent applications is based on the acid functionality of the graft polymer and of the ungrafted addition polymer. When the carboxyl groups are ionized by the addition of such a composition to an aqueous vehicle containing an amine or other fugitive base, an aqueous dispersion is produced that is water-reducible. Such dispersions are stable over long storage periods even at somewhat elevated temperatures, and remain free from gelation and precipitation. Only slight changes occur in pH levels and viscosities, indicating very little change in composition.
The reaction products of these prior applications appear to have remarkable properties. Their contents of ionized polymers are believed to serve as the means by which the ungrafted, nonionized epoxy resin component is kept in stable suspension.
For sanitary coating applications of the prior applications, Ser. Nos. 788,611 and 793,507, the preferred compositions are obtained from initial reaction mixtures in which the solids are derived 50% or more by weight from an epoxy resin having a molecular weight of at least 4,000, and the balance from addition polymerizable monomer of which the major proportion is acrylic or methacrylic acid. In a more preferred embodiment of such a sprayable sanitary coating composition, the solids of the reaction mixture are derived from an epoxy resin that contributes from 60% to 90%, and preferably about 80%, by weight of the solids, the balance being a monomer mixture of methacrylic acid, styrene, and ethyl acrylate, where the acid is the predominant monomer. Preferred sanitary coating compositions produced from such reaction mixtures have Acid Numbers (N.V.) in the range from about 80 to about 90 and preferably about 85.
While resinous coating compositions of these kinds have excellent functional characteristics and other highly desirable properties, the high content of epoxy resin increases the cost. It would therefore be highly desirable to find some way to produce functionally equivalent materials, at lower cost.
Still another important consideration is the release of solvent materials into the atmosphere. In the process of making the reaction products of the patent applications described above, it has been customary to use liquid organic solvent to facilitate handling during the manufacturing process and to improve application properties.
In the most preferred embodiment of the invention, for example, it has been customary to use two different solvents, a first solvent in which the epoxy resin, the graft polymer, and the addition polymer are all soluble, and a second solvent that can dissolve the addition polymer product and that can solvate the addition polymer side chains of the graft polymer. The solvents remain with the resinous reaction product after it is dispersed by the addition of water and a fugitive base, and more may be added to adjust application characteristics. A typical ratio of total organic solvent to total film-forming resinous solids (OS/S) is about 0.9, in a formulated, sprayable, sanitary coating composition. Consequently, when an applied coating is cured, which is usually accomplished by heating, the solvent is driven off and ordinarily escapes into the atmosphere. With the present concern over the release of organic solvent materials into the atmosphere, it is highly desirable that coating compositions be prepared in such manner as to reduce the amount of organic solvent liquid present to the smallest feasible amount.
In the fifth patent application referred to above, Ser. No. 29,106, filed Apr. 11, 1979, coating compositions and processes for making them were disclosed that advanced the art by providing compositions having the functional characteristics adverted to in connection with the invention described above in the other co-pending applications, but containing a greater proportion of addition polymer, thereby permitting economies, and also containing a smaller porportion of volatile organic solvent, thereby facilitating compliance with environmental protection laws.
Improved water-reducible coating compositions are made in accordance with this application by polymerizing in situ, in an aqueous dispersion of a resinous reaction product produced in accordance with a process of one of the earlier-filed patent applications described above, a limited quantity of addition polymerizable monomer containing ethylenic (vinyl) unsaturation, such as styrene. The result is to reduce substantially the percentage of the final composition solids represented by the initial epoxy resin, and to increase substantially the percentage of the final composition solids formed from ordinarily much less expensive addition polymerizable monomer. The resulting increase in solids and decrease in the proportion of solvent are important advantages. Moreover, the proportion of solvent may be further reduced by the addition of water during or after the in situ addition polymerization reaction. A typical OS/S ratio would be about 0.7, for a fully formulated sprayable sanitary coating composition.
The addition to the dispersed, graft polymer-containing resinous reaction product, of vinyl polymers formed by the in situ polymerization reaction, offers a still further advantage, namely, an improved set of physical and chemical characteristics for certain applications, important among which is improved resistance to weathering.